1. Field of the Invention
The present invention generally relates, but is not limited, to audio amplifiers and, more specifically, to Class AB amplifiers.
2. Description of the Related Art
A typical audio amplifier uses two complementary transistors (or groups of transistors) to drive a speaker. The amplifier might have a first transistor, which conducts current from the positive power-supply voltage to the speaker for the positive portion of the audio waveform, and a second transistor, which conducts current from the negative power-supply voltage to the speaker for the negative portion of the audio waveform. Audio amplifiers are classified according to the phase-angle portion of the 360-degree signal cycle over which current flows through each of these complementary transistors. The amplifier classes that are relevant to this specification are Classes A, B, AB, and D.
Class A amplifiers are characterized by a continuous current flow through the complementary transistors throughout each signal cycle. Class A amplifiers typically provide greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. However, the efficiency of Class A amplifiers in converting DC-source power into AC-output power is quite poor because the continuous current flow causes a relatively large portion of the DC-source power to be dissipated as heat. As a result, Class A amplifiers find most common use in small-signal applications, where linearity is more important than power efficiency. Less frequently, Class A amplifiers are used in large-signal applications, for which the need for good linearity outweighs the cost and heat disadvantages associated with the poor power efficiency.
Class B amplifiers are designed to have their complementary transistors biased near their current cutoff points, thereby causing current to flow through the transistor only during approximately 180 degrees of each signal cycle. As a result, the power efficiency of Class B amplifiers is significantly higher than that of Class A amplifiers. However, this efficiency improvement comes at the cost of substantial output waveform distortions.
Class AB amplifiers, as their designation suggests, represent a compromise between Class A and Class B types of operation. More specifically, complementary transistors in Class AB amplifiers are biased such that current flows through each transistor for less than 360 degrees, but more than 180 degrees of each signal cycle. Since practically any bias-point between the two limits corresponding to Class A and Class B, respectively, can be used in Class AB amplifiers, the latter represent a continuum ranging from lower-distortion, lower-efficiency amplifiers at one continuum end to higher-distortion, higher-efficiency amplifiers at the other continuum end.
Class D amplifiers differ from Class A, B, and AB amplifiers in that a Class D amplifier operates its complementary transistors in a switch (digital) mode, as opposed to a quasi-linear (analog) mode of operation in Class A, B, and AB amplifiers. A Class D amplifier has relatively high power efficiency because its complementary transistors are either completely turned on or completely turned off. More specifically, when the transistors are conducting (turned on), there is virtually no voltage across the transistor and, therefore, virtually no power dissipation (which is a product of voltage and current) in the transistor. Similarly, when the transistor is turned off and there is a significant voltage across the transistor, there is no current flowing through the transistor and, therefore, again there is virtually no power dissipation. An LC low-pass filter, which is usually connected to the Class D amplifier to reconstruct the analog signal from the “digital” output of the amplifier, is typically a purely reactive circuit, which ideally dissipates no power. As a result, substantially all of the output power is dissipated in the speaker for nearly 100% power efficiency, regardless of the output power level.
While Class D amplification would be a preferred approach to solving the power efficiency problem, practical implementations of Class D amplifiers suitable for an audio application encounter significant problems. For example, an all-digital implementation typically operates in an open-loop configuration, which provides no power-supply ripple rejection. As a result, any in-band power-supply ripple passes through to the speaker, which can produce an audible noise, and any high-frequency power-supply ripple aliases into the signal band by disturbing comparator thresholds. If a smoothing filter configured to remove all unwanted frequency components of the “digital” output stream is used with the amplifier, then the filter's inductors can saturate and the filter's capacitors can show polarization effects. An improved filter performance can be achieved by using specialized (and thus more expensive) parts and/or by increasing the filter's complexity and size.
A mixed analog and digital implementation of a Class D amplifier typically has a closed-loop configuration, which uses a relatively large, multi-bit analog-to-digital converter (ADC). In addition, it is often difficult to achieve stable operation of the closed-loop system due to significant nonlinearities in its feed-forward path. Further problems include (1) difficulties in implementing precision analog signal processing of the mixed analog/digital design in an integrated circuit (IC) environment, especially for a system that handles large voltages and/or currents, and (2) poor compatibility with the deep submicron technology.